Conventionally, pneumatic rubber tires are produced by molding and curing a green, or uncured, tire in a molding press in which the green tire is pressed outwardly against a mold surface by means of an inner fluid expandable bladder. By this method, the green tire is shaped against the outer mold surface which typically defines the tire's tread pattern and configuration of sidewalls. By application of heat, the tire is cured. Generally, the bladder is expanded by internal pressure provided by a fluid such as hot gas, hot water and/or steam which also participates in the transfer of heat for curing or vulcanization purposes. The tire is then usually allowed to cool somewhat in the mold, sometimes aided by added cold or cooler water to the internal surface of the bladder. Then the mold is opened, the bladder collapsed by removal of its internal fluid pressure and the tire removed from the tire mold. Such tire curing procedure is well known to those having skill in such art.
It is recognized that rubbery polymers sometimes have a tendency to stick or adhere somewhat to vulcanization or curing mold surfaces and can, therefor, be somewhat difficult to remove from the mold surface after the vulcanization procedure in the mold is completed. Sometimes the mold surface itself is treated with a release agent to reduce the sticking tendency, if present and sometimes the outer unvulcanized rubber itself (such as, for example, the sidewall and tread area of an unvulcanized tire) is coated with a coating of a release composition sometimes referred to as a precure paint, precure coating, or a precure cement. Alternatively, by more accurately designing or building the components of the tire and by more definitely designing the tire mold itself, often a precure paint and/or mold release agent is not needed. However, for some tire manufacturing purposes a tire precure coating on its outer surface is still considered desirable.
Thus, a precure coating, if used, is appropriately applied to the outside surface of unvulcanized rubbery product such as a tire prior to its molding and curing. A preferred precure coating is one which will not remain, even in part, on the surface of the mold when the rubbery product is released from the mold after vulcanization. Such mold buildup of the precure coating on the mold surface can be undesirable since it can require a periodic cleaning of the mold. An ideal precure coating should allow the portion of the rubbery product in proximity to the surface of the mold to flow without restriction and without the formation of surface defects such as cracks, air bubbles or voids in the surface of the rubber during the vulcanization step so as to result in a smooth surface. It is also desirable that the precure coating itself forms a smooth coating on the rubber product. This provides the product with a presentable appearance also may help to prevent subsequent cracking of the rubber product due to oxygen and/or ozone attack.
Various coatings have been used in the past. These include, for example, soapstone and silicone based coatings. Silicone based coating materials have been used to coat the mold or the surface of the unvulcanized rubber. The use of silicone has often been considered an improvement over the use of soapstone in that the mold release properties are usually improved.
Precure cements based on ethylene/propylene/diene terpolymer rubber (EPDM) plus another polymer (such as natural rubber and synthetic rubber) together with a curative have been used (U.S. Pat. No. 3,595,950).
Various other precure cements, or paints have been described as being useful for coating the outside surface of a tire prior to its molding and curing. For example, U.S. Pat. No. 4,329,265 describes such a paint which is water based and is comprised of a rubber latex, a wax dispersion, carbon black, water and a surfactant. While the patent remarks that various rubber latices can be used, only natural rubber or SBR latex is generally stated as being preferred and natural rubber exemplified. The paint uses a relatively large size of carbon black and a size greater than 50 millimicrons is preferred. The paint utilizes, as a necessary ingredient a wax dispersion as a means of providing slip. As it will be seen, the instant invention differs therefrom, at least in one important aspect, by specifically requiring a polybutadiene latex and, further, does not utilize a wax dispersion as a slip material.